Introduced in 1958, the 40's were powerful loco's, having a 16 cylinder English Electric diesel engine of 2000 bhp. The wheel arrangement was 1Co-Co1 (each bogie had 1idling and three driving axles) with each powered axle driven by an English Electric nose-suspended traction motor.
The engine was an English Electric 16 cylinder 16SVT diesel of 2000 bhp at 850 rpm. The loco weighed 133 tons and had a maximum speed of 90 mph.
A number of redundant class 40 locomotives were taken over by the BR engineers in the mid 1980s to haul trains required for reconstruction work on the West Coast mainline, initially around Crewe. One of these was 40012, named Aureol in connection with the clas 40s early service on the West Coast line after the 1951 Elder Dempster Lines liner. Renumbered into the engineering number series 97407. The continued use of this veteran locomotive resulted in it being saved for preservation.